Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without compromising vehicle handling while driving to where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail. Pneumatic tires designed for sustained operation when “uninflated” (“flat”) or underinflated are also called runflat tires, as they are capable of being driven in the “flat” condition. They are also called extended mobility technology tires, or EMT tires. A conventional pneumatic tire will collapse upon itself when it is uninflated and carrying the weight of a vehicle. The tire's sidewalls buckle outward in the circumferential portion of the tire where the tread contacts the ground, making the tire “flat.”
The term “runflat” is generally used to describe a tire that is designed such that the tire structure alone, and in particular the structure of the sidewalls, has sufficient strength and rigidity to support the vehicle load when the tire is operated without being inflated. The sidewalls and internal surfaces of such runflat tires or EMT tires do not collapse or buckle due to their rigidity, and the prevailing designs of such tires do not otherwise contain or use other supporting structures or devices to prevent the tire from collapsing. Examples of such other supporting structures are devices that might be contained within the tire and which experience no loading during normal inflated operation.
In general, runflat tires or EMT tires incorporate sidewalls that are thicker and/or stiffer so that the tire's load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed wedge inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, each sidewall is thickened in the region between the bead and the tread shoulder. The wedge inserts in each sidewall are generally crescent-shaped in cross-sectional view, in order to conform to the shape of the sidewalls. Such wedge reinforced sidewalls, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall that is closest to the road-contacting portion of the tread. More specifically, the bending stresses on the sidewalls are such that the axially outwardmost portions of the reinforced sidewalls experience tensile stresses while the axially inward portions experience compressive stresses during runflat operation.
A Goodyear patent U.S. Pat. No. 5,368,082 ('082), by Oare et al, discloses a low aspect ratio runflat pneumatic radial ply tire which employs multiple wedge inserts in each sidewall to improve runflat stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb. load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty which could, however, be partially offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios. The '082 patent teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
Two U.S. Pat. Nos. 5,427,166 and 5,511,599 of Walter L. Willard, Jr., show Michelin tires that incorporate an additional third ply and a third insert in the sidewall to further increase the runflat performance of the tire over that of the '082 patent. These Willard patents discuss some of the load relationships that occur in the uninflated condition of the tire and demonstrate that the concept taught in the '082 patent can be applied to additional numbers of plies as well as additional wedge inserts in each sidewall.
However, such large amounts of rubber used to stiffen the sidewall members become factors in flexure heating that leads to tire failure during runflat operation. This is especially so when the tire is operated at high speeds during low or zero inflation. Therefore, one goal of runflat tire design is to minimize the number of wedge inserts used to stiffen each sidewall and the total amount of wedge insert material used in runflat tire.
While the high resistance to compression and deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall has drawbacks which include the above mentioned increase in tire weight and flexure-induced heat buildup. Such designs also increase the tire's complexity in ways that adversely affect manufacturing and quality control.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives both good runflat vehicle handling as well as good service life during runflat operation.